According to FIG. 1, a rotor 101 of a conventional electrical machine 100 comprises α. plurality of magnetic rotor poles 102 uniformly divided along a rotor perimeter. The angular distances between two adjacent rotor poles (centre lines) are called pole pitches 103, and they all have an equal value of α. Similarly, a stator 110 of a conventional electrical machine 100 comprises a plurality of uniformly distributed stator teeth 111 separated by stator slots 112. Conductors 113 are extending in the stator slots, the angular distances between two adjacent conductors (centre lines) being called coil pitches 114. In a conventional electrical machine according to FIG. 1 the conductors are thereby distanced by uniform coil pitches 114 with a value β.
A rotor with permanent magnetic poles exhibits torque ripple caused by a cyclic torque called cogging. Cogging results from the tendency of the rotor and the stator to align themselves in a position of minimum magnetic reluctance, and this phenomenon exists even in an un-energized machine. Cogging occurs when the rotor poles are moving over the edges of the stator teeth. The magnitude of cogging is the sum of interaction between individual rotor poles and stator teeth, and it depends on the relationship between the number of rotor poles and stator teeth.
The cogging problem is conventionally addressed by adjusting rotor poles. In permanent magnet machines for example, when rotor poles comprise several magnets arranged in rows extending in an axial direction, it is known to skew the individual magnets in relation to one another in circumferential direction. It is also known to shape the permanent magnets in order to make the moving over a stator tooth edge smoother. Furthermore, it is known to adjust the angular distances of adjacent rotor poles such that the distances become non-uniform. Corresponding adjusting measures can also be applied to the stator teeth, but many times the adjustment of rotor poles is practically more feasible than the adjustment of stator teeth.
U.S. Pat. No. 4,751,416 discloses a permanent magnet motor with non-uniformly distributed magnetic poles on the rotor. The magnetic poles are displaced through an angle corresponding to a fraction of a stator slot pitch i.e. the angular distance between adjacent stator tooth edges. In some embodiments of U.S. Pat. No. 4,751,416 the magnetic forces acting on the rotor are not in balance, while in the other embodiments magnetic forces are balanced out. In the balanced embodiments the magnetic poles are effectively adjusted pair-wise.
U.S. Pat. No. 6,285,104 discloses a permanent magnet motor and a synchronous reluctance motor with non-uniformly distributed magnetic poles on the rotor. U.S. Pat. No. 6,285,104 further discloses a stator with non-uniformly distributed stator teeth. According to all embodiments of U.S. Pat. No. 6,285,104 the magnetic poles or the stator teeth are displaced such that the magnetic forces acting on the rotor are not in balance.
Unbalanced magnetic forces are clearly not desirable since the unbalance causes vibrations that may become unacceptably strong. It is also not desirable to adjust the rotor poles pair-wise because of the complexity of this adjustment from the manufacturing point of view. There is therefore a desire to provide a magnetically balanced alternative to the pair-wise adjustment of the magnetic poles.
JP8251847 discloses a permanent magnet machine with non-uniformly distributed magnetic poles on the rotor. The poles are adjusted in a balanced manner by symmetrically adjusting the circumferential positions of groups of poles. The rotor consists of a single non-divided segment.
JP2005137117 discloses a permanent magnet machine wherein the rotor is divided into a plurality of discrete segments.
JP3284148 discloses an electrical machine wherein a cogging torque reduction is achieved by shifting groups of stator pole shoes in clockwise or anticlockwise direction while maintaining the coils at uniform circumferential distances.